1. Field of the Invention
The present invention relates generally to prosthetic hearing devices, and more particularly, to a bone conduction hearing devices generating stimulation via tangentially-directed vibrational force with respect to a surface of the recipient's bone.
2. Related Art
There are three basic types of hearing loss: sensorineural, conductive, and mixed hearing losses. Sensorineural hearing loss results from damage to the inner ear or to the nerve pathways from the inner ear to the brain. The majority of human sensorineural hearing loss is caused by abnormalities or damage to the hair cells in the cochlea. Hair cells in the cochlea are the sensory receptors that transduce sound to nerve impulses. Acoustic hearing aids may be appropriate for those who suffer from mild to moderate sensorineural hearing loss. In cases of severe or profound sensorineural hearing loss, a cochlear implant may be the appropriate choice. Cochlear implants bypass the hair cells in the cochlea and directly stimulate the auditory nerve fibers in the cochlea by an electrode array that is implanted in the cochlea. Simulation of the auditory nerve fibers creates the sensation of hearing in the recipient.
Conductive hearing loss occurs when there is a problem with the conduction of sound from the external or middle ear to the inner ear. This type of hearing loss may be caused by anything that impedes the motion of the ossicles, the three bones of the middle ear that conduct sound to the cochlea. It may also be caused by a failure of the eardrum to vibrate in response to sound or fluid in the middle ear. Conductive hearing loss may be treated by acoustic hearing aids, middle ear implants, and the like.
Still other individuals suffer from mixed hearing losses, that is, conductive hearing loss in conjunction with sensorineural hearing. In other words, there may be damage in both the outer or middle ear and the inner ear (cochlea) or auditory nerve.
While many individuals suffering from conductive hearing loss often use acoustic hearing aids, such hearing aids may not be suitable for all individuals, such as those suffering from chronic ear infections or from single-sided deafness. An alternative treatment is the use of bone conduction hearing aids, or simply conduction devices herein.
Bone conduction hearing aids utilize the bones of an individual's bone to transmit acoustic signals to the cochlea. Generally, most bone conduction hearing aids function by converting a received sound signal into vibration. This vibration is then transferred to the bone structure of the bone, in one particular embodiment the skull. This skull vibration results in motion of the fluid of the cochlea, thereby stimulating the cochlear hair cells and causing the perception of sound in the recipient.
Bone conduction devices may be attached to a titanium implant implanted in a recipient's bone, via an external abutment. In one particular embodiment of the present invention, the titanium implant is surgically implanted into the part of the skull bone that is behind the ear and allowed to naturally integrate with the skull bone over time. The bone conduction device is coupled to the titanium implant via the external abutment. Vibrations from the bone conduction device are then transmitted to the skull through the external abutment and the titanium implant to stimulate nerve fibers of the inner ear of the recipient.
Some bone conduction devices produce sound perception by applying a vibrational force directly to the recipient's bone, which is communicated through the bone eventually to the cochlea where the fluids contained therein are vibrated. In some devices, the vibrational force is directed towards the recipient's bone perpendicularly with respect to the surface of the recipient's bone. In such devices, the angle between the surface of the recipient's bone and the direction of the vibrational force is as close to 90 degrees as possible in order to ensure the most efficient transfer of vibrational force to the recipient's bone as possible. As noted earlier, the transferred vibrational force is conducted through the bone to the recipient's cochlea, causing motion of the cochlear fluid, thereby producing sound perception. It may be possible to cause that movement of cochlear fluid to produce the sound perception in other ways without directly applying and communicating a vibrating vibrational force to the recipient's bone.